1. Field of the Invention
A two-stage hydraulically or pneumatically actuated piston for providing a variable thrust force as a function of piston rod extension.
2. Discussion of Related Art
Many machine applications require a very large initial breakaway force but with a much lesser force being needed later in an operating cycle. In an exemplary situation, such as encountered when using the Brush Clearing Device disclosed in co-pending U.S. patent application Ser. No. 08/383,011, filed Feb. 3, 1995, in the name of the inventor of this invention, and now U.S. Pat. No. 5,526,637. In that invention, a hinged-jaw assembly is secured to a load lifting means, such as a hydraulic piston, mounted on a farm tractor. The jaws of the assembly are caused to enclose a swath of brush in a tight embrace whereupon the hydraulic lifting means raises the jaw assembly to pull the brush out of the ground by the roots. Thereafter, the load is raised to a height to allow deposition of the uprooted brush in, for example, a stake-body truck. Uprooting the brush requires a great deal of initial force. But once the brush is plucked from the ground, while the force required to maneuver the load of brush above the truck bed is minimal, a considerable lift extension is needed.
Thus, there is a need for applying a large initial force over a short distance followed by subsequent application of a lesser force over a much longer distance.
FIG. 1 is a common form of a prior art device for accomplishing the above desideratum using a telescoping piston assembly. Either hydraulic or pneumatic power can be used to operate the piston. A large-area piston 10 is contained within a cylinder 12. Piston 10 includes a hollow piston rod 14. Hollow piston rod 14, in turn, becomes the cylinder for containing a second piston 16 whose exposed area is less than the area of piston 10. Piston rod 18 is secured to the device to be actuated. In operation, pressurized fluid that is applied through inlet port 20 first moves piston 10 to the end of its stroke, whereupon, piston 16 continues its extension until it abuts the end of hollow piston rod 14. To retract the pistons, if they are not to be retracted by gravity, hydraulic fluid must first be introduced through inlet port 22 to retract piston 10 and thereafter fluid must be introduced through inlet port 24 to retract piston 18, thus requiring a rather complex hydraulic circuit.
More than two nested pistons can be used if desired. Telescoping pistons are sometimes used in situations where the assembly must be relatively compact when the pistons are retracted but, nevertheless, a long stroke is needed.
The disadvantages to the arrangement of FIG. 1 are manifold. There are two or more polished exposed piston rods, 14 and 18, that must be protected from corrosive ambient environments. There are many surfaces that need to be polished and accurately concentered. Furthermore, the retraction operation is complicated by the requirement for at least two separate valving system, one for each piston 10 and 18, or more valving if multiple nested pistons are used.
U.S. Pat. No. 4,828,230 teaches a Dual Acting Hydraulic Actuator for Active Suspension System, issued May 9, 1989 to C. B. Stegar et al. The system provides an active suspension for vehicles with a dual acting hydraulic actuator providing for shortened overall length for a given amount of stroke. The cylinder tube of the actuator makes use of concentric tubes proportioned to provide the same pressure/force relationship in both directions. This case is cited to show a version of a telescoping actuator used to minimize the total length of the device for a preselected length of stroke.
U.S. Pat. No. 4,928,733, issued May 29, 1990 to J. M. Sudolnik et al. provides a Steam Valve with Variable Actuation Forces. This valve is essentially a dual element telescoping piston configured as a pilot valve that contacts a small-diameter valve seat which is operative during a cracking cycle in order to maintain positive control of the valve. Later a larger-diameter main valve is opened to reduce the actuation forces required beyond the cracking position. This reference is cited to show use of a telescoping piston to provide a variable force.
U.S. Pat. No. 4,341,147 issued Jul. 27, 1982 to R. E. Mayer for a Coaxial Hollow Piston Regenerative Liquid Propellant Gun, teaches use of a multi-element combustive/hydraulic gun-firing system which is essentially a complex version of the telescoping piston of FIG. 1 of this disclosure. The system includes a first coaxial pumping piston which is a differential area pressure piston operating between a combustion chamber and the primary propellant reservoir. A second coaxial piston in a bore in the first piston opens and closes injection ducts running through the pumping piston from the primary reservoir to the bore to interdict flow of propellant to the combustion chamber. This reference is cited to illustrate another application of telescoping coaxial pistons.
There is a need for a fluidic piston-type actuator of simple design that is capable of applying a force that is a function of the piston extension.